Allow a per-jobset check interval

This allows checking a jobset (say) at most once a day.  It's also
possible to disable polling by setting the interval to 0.  This is
useful for jobsets that use push notification or are manually
evaluated.

src/sql/upgrade-12.sql

@@ -1,2 +1 @@
-
 create index IndexBuildOutputsOnPath on BuildOutputs(path);